The prevention of ventricular tachycardia and other dangerous arrhythmias has become a subject of intensive study and research. Ventricular tachycardia (VT) is a lethal arrhythmia which is known to frequently lead to sudden death, usually after progressing to ventricular fibrillation (VF). As a result, there have been many investigations into techniques for monitoring a patient's cardiac condition to determine when conditions indicate a likelihood of VT, and to otherwise continually assess the risk of VT and VF.
It is known that QT interval, and in particular abnormalities of the QT interval, are associated with ventricular arrhythmias, e.g., in long QT syndromes and after myocardia infarction. See "Diurnal Variation of the QT Interval--Influence of the Autonomic Nervous System," Bexton et al., British Heart J., 1986; 55:253-8. Thus, the QT interval of the surface EKG is accepted as an indirect measure of patient myocardial depolarization and repolarization. It is known that QT is longer during sleep, and in fact QT corrected for rate (QTc) is also longer during sleep. Further, it has been noted that QTc interval and QTc variability reach peak shortly after awakening hours, which may reflect increased automatic instability during early waking hours; and further that the time of the peak value corresponds to the period of reported increased vulnerability to ventricular tachycardia and sudden cardiac death. Molnar et al., J Am. Coll. Cardiology, 1996 Jan., 27:1, 76-83. See also Christenson et al., PACE, Vol. 19, September 1996, 1296-1393, stating that QTc exhibits significant diurnal variability. The literature suggests that there are significant changes in the autonomic system during sleep, with either an increase in para-sympathetic tone, or an increase in sympathetic activity, or both. There is also a diurnal variation in circulating catecholamines, and specifically the catecholamine level drops during the night. These factors are known to influence repolarization, nighttime variations of which are reflected in the lengthening QT interval, and lengthened QTc. Other investigations have looked into the temporal and spatial distribution of QT intervals, and suggest that QT dispersion (QTd), measured as the difference between the maximal and minimal value of QT duration, can be associated with increased risk of ventricular tachycardia and sudden cardiac death. Accordingly, the literature presently suggests that both QT interval prolongation and increased QT dispersion (refractory dispersion), or dispersion of the repolarization duration, are important reflectors of the risk of an incipient ventricular tachycardia.
This invention is responsive to the need to address the increased vulnerability to attacks of VT during or just after the waking period, and utilize the fact that the awakening period is characterized by observable variabilities of the depolarization and repolarization waves and the QT duration. In particular, the invention meets the need for a pacemaker which incorporates the capacity to monitor the diurnal heart pattern, particularly changes of the QRS-T waveform during the awakening period, so as to provide an indication of a risk of VT. A pacemaker in accordance with this invention also automatically provides for a controlled pacing response designed to prevent the onset of VT during the vulnerable awakening period.